Flexible readout and signal processing in a computational sensor array

ABSTRACT

A computational sensing array includes an array of sensing elements. In each sensing element, a first signal is generated from a transducer. A second signal is produced by a collection unit in response to receiving the first signal. The second signal may be modified, in a conditioning unit. A sensing element preprocessing unit generates a word representing the value of the modified second signal, and may produce an indication of change of the first signal. A current value of the word may be stored in a state holding element local to the sensing element, and a previous value of the word may be retained in a further state holding element local to the sensing element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of prior-filed, co-pending U.S.Nonprovisional Application No. 13/671,146, filed Nov. 7, 2012, whichclaims priority to and the benefit of prior-filed U.S. ProvisionalApplication Nos. 61/556,612 and 61/556,616, both filed Nov. 7, 2011, thecontents of all of which are herein incorporated by reference in theirentireties.

STATEMENT OF GOVERNMENT INTEREST

Certain research which gave rise to portions of this invention was madewith government support under contract number HQ0006-07-D-0001 awardedby the Missile Defense Agency (MDA). The government may have certainrights in the invention.

BACKGROUND

A Sensing Array (SA) is an array of sensing elements at or near thesurface of transducing elements. One type of sensing array is a focalplane array (FPA), which has sensing elements arranged at the focalplane of a lens or a mirror. Sensing arrays can be used in imaging, aswith light sensors in visible, infra-red and ultra-violet imaging, inelectrochemical sensing arrays, or other electromagnetic sensing arrayssuch as magnetic field sensing or terahertz imaging. Properly biasedsensors each typically convert the sensing quantity into a voltage,current, charge or resistance related to the physical quantity incidenton the sensor. Charge coupled devices (CCD) are often used for visibleimagers. Infrared (IR) imaging often makes use of infrared sensors and aseparate chip called a readout integrated circuit (ROIC), which arebonded together in an assembly as a focal plane array. Sensing arrayssuch as focal plane arrays or readout integrated circuits may havesensors integrated with circuitry in a system-on-chip. A ComputationalSensing Array (CSA) is an array of computational structures at, near orincluding a sensing array (SA).

Responses of individual elements in the array can exhibit variability asa result of differences in the sensors in a specific array. Variabilityincludes, but is not limited to sensors having different zero-signallevels and/or different gains. Variability that is time-varyingmanifests itself as temporal or spatial noise. Stationary spatialvariability/noise is often called non-uniformity. This can be correctedacross the array of pixels by applying a variability correction.Time-dependent variability can also be introduced in the process ofquantizing the continuous time, continuous value signal to yield acontinuous or discrete time discrete value signal. Generally, acorrection process is applied using a processor and software, forexample a digital signal processor (DSP), as a postprocessing operationafter the individual sensing elements have been read out to a memoryexternal to the sensing array. Applying variability correction, forexample non-uniformity correction (NUC), as a postprocessing operationusing a processor and software is common practice in sensor arrays,including imaging arrays that are scanned such as CMOS or CCD imagers.Doing the latter is energy inefficient and resources inefficient andcannot always be performed, especially in event-based sensor arrayswhere response is necessary at time scales beyond those involved withsoftware-based NUC correction. There is a need in the art for a solutionwhich overcomes the drawbacks described above.

SUMMARY

Embodiments of a sensing array and a method for transducing in a sensingarray are herein disclosed. The sensing array may be implemented as anelectronic circuit, an integrated circuit, a system-on-chip, a readoutintegrated circuit or a hybrid in electronic, mechanical, chemicaland/or bio-physical arrays. Embodiments may have sensors integratedon-chip, bonded or otherwise connected to the circuitselectromagnetically or electrochemically coupled to the array of thechip.

Some embodiments include a Computational Sensing Array (CSA) electroniccircuit. The circuit includes an array of sensing elements. Each sensingelement unit has a transducer, a collection unit, a conditioningstructure and a sensing element preprocessing unit. The transducer isconfigured to produce a first signal in response to a physical signal.The collection unit is coupled to the transducer. The collection unit isconfigured to produce a second signal derived from the first signal. Thesecond signal represents the transduced physical quantity and it couldbe of the continuous-time/continuous-value, orcontinuous-time/discrete-value type. The conditioning unit is coupled tothe collection unit. The conditioning unit is configured to modify thesecond signal. The sensing element preprocessing unit is coupled to theconditioning unit. The sensing element preprocessing unit is configuredto produce a value of the second signal as modified. The sensing elementpreprocessing unit is further configured to produce an indication ofchange of the first signal, which may represent a change in theenvironmental condition or a time varying change within the transducer.

Some embodiments include a computational sensing array that may be usedfor imaging. The computational sensing array includes an integratedcircuit with an array of sensing elements. Each sensing element has acollection unit, a conditioning unit and a space to time multiplexingunit. The collection unit is operable to be coupled to a transducer. Thecollection unit is configured to produce acontinuous-time/discrete-value or discrete-time/discrete value signal,based on activity of the transducer. The conditioning unit is coupled toreceive the continuous-time/discrete-value or discrete-time/discretevalue signal. The conditioning unit has a state-holding and collectorelement of continuous-time/discrete-value or discrete-time/discretevalue signal type, which may be an accumulator or an integrating stateholding element. The state-holding and collector element is configuredto produce an adjusted value from the state-holding and collectorelement of the continuous-time/discrete-value or discrete-time/discretevalue signal. The space to time multiplexing unit is configured tooutput the adjusted value.

Some embodiments include a method for transducing in the sensing array.The sensing array includes a plurality of sensing elements. Actions ofthe method take place in each sensing element. The method includesgenerating a first continuous-time/continuous-value signal from atransducer. Then, in response to receiving the firstcontinuous-time/continuous-value signal, the method includes generatinga signal that can be modulated by a discrete or continuous time signal.The discrete or continuous time modulated signal represents a value ofthe first continuous value continuous time signal. For example, in oneembodiment a count of pulses of a modulated signal over a specifiedperiod of time represents a value of an analog signal from which themodulated signal is derived. Method steps include applying an offsetand/or a gain to a value of the modulated signal. The method includesgenerating a multibit Boolean binary or non-Boolean symbolicrepresentation of the signal, i.e., a vector. The latter represents avalue of the modulated signal with the applied offset or gain. Themethod includes storing the present value/state of the multibit symbolicrepresentation in a first array of state holding elements of the propertype (continuous-time/discrete-value or discrete-time/discrete valuesignal). A previous value of the multibit symbolic representation, i.e.,a value from a previous frame or epoch, is retained in a second array ofstate holding elements. The first and second arrays of state holdingelements are local to the computational sensing element.

Other aspects and advantages of the embodiments will become apparentfrom the following detailed description taken in conjunction with theaccompanying drawings which illustrate, by way of example, theprinciples of the described embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The described embodiments and the advantages thereof may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings. These drawings in no waylimit any changes in form and detail that may be made to the describedembodiments by one skilled in the art without departing from the spiritand scope of the described embodiments.

FIG. 1A is a block diagram of a high level architecture of aComputational Sensing Array (CSA).

FIG. 1B is a perspective view of multiple computational arrays inlayers, as used in an embodiment of the Computational Sensing Array ofFIG. 1.

FIG. 2 is a block diagram of a high level architecture of acomputational sensing element (CSE) in the CSA of FIG. 1A.

FIG. 3 is a block diagram of an architecture of a CSE and anarchitecture of a CSA, in accordance with an embodiment of the presentinvention.

FIG. 4 is a combination circuit and block diagram of a CSE in anembodiment of the CSE architecture of FIG. 3.

FIG. 5 is a group of circuit diagrams, each showing an embodiment of acircuit useful in continuous-time/discrete-value ordiscrete-time/discrete value signal in FIG. 4.

FIGS. 6A-C are circuit diagrams of embodiments of a front endintegrating amplifier or integrator and transducer that can be used inan embodiment of the CSE architecture of FIG. 3.

FIG. 7 is a flow diagram of an embodiment of a method for sensing in aCSA.

FIG. 8 is a block diagram of an embodiment of the Computational SensingArray of FIG. 1, having a Homeostasis Processor Unit.

DETAILED DESCRIPTION

Embodiments of a computational sensor array as described herein employ asensor system architecture with time varying (non-ergodic) compensationfor temporal and spatial variability (i.e., conditioning) in eachsensing element (e.g., pixel for light sensing applications), andrelevance indexing to decrease the bandwidth, processing and/orcomputing burden of computing and/or other devices external to thesensor array. Particularly, transmission and postprocessing of arraydata is decreased. In addition, it allows for multi-mode operation suchas passive and/or active as well as multi-band or multi-color. The arrayin this case becomes a type of sensor system-on-chip. Some embodimentshave transducers integrated with circuitry on the same chip. Someembodiments have transducers flip chip bonded to the circuits layermaking a focal plane array (FPA). Other embodiments include multiplelayers as 3-D structures where each tier can apply different technologyand may involve solid-state and other sensing arrays. Other mounting andbonding arrangements of sensors and the computational sensing array maybe devised.

Detailed illustrative embodiments are disclosed herein. However,specific functional details disclosed herein are merely representativefor purposes of describing embodiments. Embodiments may, however, beembodied in many alternate forms and should not be construed as limitedto only the embodiments set forth herein. Specifically, some embodimentsare described in terms of pixels, pixel units and a focal plane array ofpixel units, while other embodiments are described in terms of sensingelements and sensing arrays with arrays of sensing elements, orcomputational sensing arrays. Embodiments of pixel units may serve asembodiments of sensing elements, and embodiments of focal plane arraysmay serve as embodiments of sensing arrays. Some embodiments aredescribed in terms of registers. A register, it should be appreciated,is an example or a subset of the class of state holding elements. Someembodiments are described in terms of sensors or detectors, which areexamples of transducers. An accumulator is herein used as a type ofintegrating state holding element. A frequency modulated signal is atype of modulated signal. A frame is a type of epoch. Autoscanning is atype of autonomous operation. Analog signals are considered a subset ofcontinuous time, continuous value signals. A binary-valued signal is atype of discrete-valued signal. Priority is a type of relevance. Amultibit word is an example of a type of vector.

It should be understood that although the terms first, second, etc. maybe used herein to describe various steps or calculations, these steps orcalculations should not be limited by these terms. These terms are onlyused to distinguish one step or calculation from another. For example, afirst calculation could be termed a second calculation, and, similarly,a second step could be termed a first step, without departing from thescope of this disclosure. As used herein, the term “and/or” and the “/”symbol includes any and all combinations of one or more of theassociated listed items.

As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”,“comprising”, “includes”, and/or “including”, when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. Therefore, the terminology usedherein is for the purpose of describing particular embodiments only andis not intended to be limiting.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

In the embodiment of a Computational Sensing Array 100 shown in FIG. 1A,detection, collection, conditioning, preprocessing and readout are allperformed at the CSA level. This contrasts with known architectures ofCCD or CMOS imaging FPAs, in which detection is done at the pixel level,collection is performed at the row or column level, readout is performedat the row or column level, and conditioning and postprocessing areperformed by a computing device external to the CCD or CMOS FPA device.

In some embodiments the Computational Sensing Array 100 shown in FIG. 1Aincludes physically multiple CMOS silicon computational arrays in layersinterconnected vertically through interconnects, so that the multiplearrays are in close physical proximity to each other (see FIG. 1B).

In some embodiments, each pixel in Computational Sensing Array 100 has adetection unit 102, in which a sensor, detector or transducer receivesphotons. Biasing to the sensor, detector or transducer, applied in thepixel (i.e., the sensing element), allows the sensor to show a change involtage, current, charge or resistance in response to receiving thephotons. This response can then be collected so that a value can bederived, relating to the detected photons.

In some embodiments of the Computational Sensing Array 100 for lightsensing applications, each pixel in a focal plane array (or sensingelement in a sensing array) has a collection (i.e. conversion andstorage) unit 104, in which the response of the sensor or detector iscollected and converted to some type of signal. The signal may be acontinuous time, continuous value signal, an analog signal such as atime varying voltage or current, a digital signal such as a multibitdigital word or vector, a discrete value continuous time signal such asa frequency modulated binary valued signal, or other type. In oneembodiment, the collection unit has an analog to digital converter. Inother embodiments, the collection unit has a frequency modulatinganalog-to-digital converter with a digital counter. In alternativeembodiments, the collection unit has a frequency modulatinganalog-to-digital converter with a digital counter and gain control.

In some embodiments, each sensing element (or pixel) in theComputational Sensing Array 100 has a time varying conditioning orcompensation unit 106, in which the collected response of the sensor ordetector is modified or adjusted for temporal and/or spatialvariability, i.e., conditioned or compensated. In one embodiment, thetime varying compensation unit has a nonuniformity correction, which isperformed on the pixel prior to readout of the pixel, i.e., is performedlocal to the pixel as preprocessing in the array.

In some embodiments, each sensing element in the sensing array 100 (orpixel in the focal plane array) has a preprocessing unit 108, in which atype of preprocessing is applied to the pixel prior to readout of thepixel. In one embodiment, the preprocessing unit has a pixel changedetection. In one embodiment, the preprocessing unit cooperates with achange detection module that is column-based or row-based.

In some embodiments, each sensing element in the sensing array 100 (orpixel in the focal plane array) has a preprocessing unit 108, in whichthe mean and variance of the sensing element (or pixel) is calculated.In one embodiment, the preprocessing unit cooperates with a mean and/orvariance discrimination module that is column-based or row-based.

In some embodiments, each sensing element in the sensing array 100 (orpixel in the focal plane array) has a relevance indexing unit that rankseach sensing element (i.e., pixel for light sensing applications) forreadout (i.e., output of the array) unit 110. This relevance unitautomatically places the sensing elements or pixels deemed of intereston the data bus to be accessed by the external device. The data includesthe conditioned sensing element or pixel values or intermediate signalvalues and the unique address and time stamp. A processor or otherdevice external to the focal plane array can use the sensing element orpixel address to read out the sensing elements or pixels, i.e., read thecollected, conditioned, i.e., compensated, and/or preprocessed pixelvalues, by sending the unique address and a read request to the focalplane array (or sensing array 100). Row decode and column decodecircuitry is applied to route the read data out from the pixel readoutunits to a data bus in a manner similar to a memory read. Multiple suchcircuits may be present in order to read out multiple pixelssimultaneously. Further embodiments of the Computational Sensing Array100 have various combinations of the above-described units, with one ormore units deleted from, substituted in or added to each pixel. Someembodiments have the ability to readout any intermediate signal value,and in one embodiment to read the raw sigma-delta bitstream, from anypixel or sensing element.

An embodiment of the architecture of a sensing element (or pixel unit)is shown in FIG. 2. A sensor 202 is coupled to an analog-to-digitalconverter 204 and a time to voltage converter 206. An external trigger212 starts the time to voltage converter 206, which produces a sensedvoltage relating to an environmental condition sensed by the sensor 202,after a controlled time interval. In one embodiment, the time to voltageconverter includes an integrator which integrates current or chargeflowing to or from the sensor 202. The analog-to-digital converter 204produces a digital output as a conversion of the sensed voltage from thetime to voltage converter 206. In one embodiment, the analog-to-digitalconverter 204 includes a sigma delta modulator, which produces afrequency modulated binary valued signal representing the sensed valuefrom the sensor 202.

Output of the analog-to-digital converter 204 is received by thecomputational structure 210. In one embodiment, the computationalstructure 210 includes an arithmetic logic unit. The computationalstructure 210 is coupled to a memory 208. In one embodiment, thecomputational structure combines the converted value from theanalog-to-digital converter 204 with data from the memory, therebymodifying the sensed value. The modified sensed value is then availableat the data output of the computational structure 210.

Moving down one level, architecture of a Computational Sensing Array 300and a sensing element 350 in the Computational Sensing Array 300 areshown in FIG. 3. Embodiments of the Computational Sensing Array 300include embodiments of focal plane arrays, and embodiments of thesensing element 350 include embodiments of pixels and pixel units. Thesensing array 306 (which may be embodied as a pixel array) includes anarray of sensing elements 316 (which may be embodied as pixel units),one of which is shown for illustration. A row decoder 302 and a columndecoder 314 are used to address the sensing elements or pixels asappropriate, in a manner comparable to a standard memory, e.g., arandom-access memory (RAM). An I/O block 304 is used for data input andoutput to various blocks and the sensing array 306, in a mannercomparable to standard I/O blocks of peripherals. Additional blocks ofthe Computational Sensing Array 300 will be discussed following thedescription of the sensing element 350 architecture.

In one embodiment, activity of a sensor is converted from analog todigital. In another embodiment, activity of a sensor is converted from acontinuous value, continuous time signal (of which class an analogsignal is a subset) to a digital value. In the embodiment of a sensingelement 350 shown in FIG. 3, analog-to-digital conversion of sensoractivity is performed in part by a sigma delta modulator 354. The sigmadelta modulator 354 produces a pulse train with a time varyingfrequency, for example, a binary value analog time signal or a frequencymodulated signal. The number of pulses produced during a specified timeinterval is a function of the analog voltage, current or charge seen atthe output of the sensor. This characteristic operation of a sigma deltamodulator is known as pulse density modulation or pulse frequencymodulation. Conversion to a digital value consists of counting thepulses during the specified time interval. This could be performed inone embodiment without gain modification by using a standard binarycounter with a sufficient number of bits to avoid overflow with amaximum count. The binary counter would be initialized to zero, andcount pulses by incrementing with each pulse, e.g., by using the pulsesignal as a clock for an incrementing counter or by using the pulsesignal as a count enable.

In the embodiment shown in FIG. 3, the sensing element 350 includes again register 352, the sigma delta modulator 354, an offset register356, and an accumulator 362. In one embodiment, the accumulator 362includes a fixed point adder or other computational structures. The gainregister 352 is read/write accessible and stores a gain value which canbe less than one, equal to one or greater than one. In other words, thegain value has a range spanning to both sides of unity gain, inclusive.In one embodiment, the gain value is stored as a fixed point number.Further embodiments can use floating point arithmetic, and binary orbinary coded decimal numbers. The offset register 356 is read/writeaccessible and stores an offset value which can be less than zero, zero,or greater than zero. In other words, the offset value has a rangespanning to both sides of zero offset, inclusive. In one embodiment, theoffset value represents the dark current or dark voltage of a sensor,which relates to the activity of the sensor when no light is incident.In another embodiment, the gain value represents the slope of the sensorresponse to light incident on the sensor. As each sensing element orpixel has a respective gain register 352 and a respective offsetregister 356, the sensors can be calibrated and individual differencesaccounted for across the array, with the sensing elements or pixelshaving individual adjustment to the sensed values accordingly.

In some embodiments, the offset and/or gain are modified in a timevarying manner based on sensing element or pixel, and/or local, and/orglobal characteristics, as determined by an external controller or byadditional circuitry internal to the sensing element or pixel or thecomputational sensing array.

Modification of the sensed value, in each sensing element or pixel,proceeds as follows. At reset, the offset register 356 value is placedinto the accumulator 362. This action is controlled by the multiplexer360, which can select a multibit value from the output 374 of the offsetregister 356, or can select the output 376 of the accumulator 362.Further embodiments can be readily devised for loading the value of theoffset register into the accumulator 362. When reset is released,counting of pulses from the sigma delta modulator 354 proceeds. Itshould be understood that the reset signal may be an externally orinternally generated signal.

When the output of the sigma delta modulator 354 is zero, i.e., no pulseis present, the multibit output of the AND gate 358 is zero, and theaccumulator 362 retains the previous accumulated value. When the outputof the sigma delta modulator 354 is a binary one, i.e., a pulse, themultibit output of the AND gate 358 is equal to the multibit output 370of the gain register 352, and the accumulator adds the gain value to theprevious accumulated value. The accumulator thus increments by the gainvalue with each pulse from the sigma delta modulator 354. The gain valueis an incremental value added, i.e., counted, in response to each of aplurality of pulses from the sigma delta modulator. In some embodiments,a specified interval timer may control the length of time over which thepulses are counted. In one embodiment, the interval timer isprogrammable. In some embodiments, the sigma delta modulator may be ofthe multibit type, in which case the output is multiplied (rather thanANDed) with the gain register output.

It should be appreciated that, by using the gain value and the offsetvalue in the above-described manner, the sensing element or pixelapplies an offset and a gain as a modification or a correction to thesensed value from the sensor. Two-point nonuniformity correction isachieved by starting the accumulator at a non-zero value, for offsetcorrection, and incrementing by a fractional value greater or less thanone, for gain correction. In one embodiment, an ideal sensor wouldreceive a zero offset and a unity gain. It should also be stated that amore sophisticated nonuniformity correction can be implemented (e.g.4-point NUC)

Further embodiments contain multiple gain registers, such that theparticular gain value used by the accumulator is controlled by anexternal or internal signal. In this manner, the set of gains can beswitched instantaneously in order to, for example, account for a changein the environment, sensor biasing, sensor type, or to facilitatesynchronous detection of weak signals.

The output 376 from the accumulator 362 can be selectively latched ineither of two output registers 366, 368, as directed by a registerselect 378 and a demultiplexer or selector 364. In one embodiment, oneof the output registers 366, 368 stores the most recent sensing elementor pixel value and the other of the output registers 366, 368 stores theimmediately preceding sensing element or pixel value or another prior orpreceding value of the sensing element or pixel. Further embodiments,such as storing multiple preceding values or storing only the mostrecent sensing element or pixel value, are readily devised. It should beappreciated that one output register 366 can be read out by an outputport 380, and the other output register 368 can be read out by a furtheroutput port 382.

Further embodiments contain multiple accumulator circuits within thesensing element or pixel controlled by additional reset signals, so thatthe respective time intervals are longer, shorter, and/or overlapping.It should be understood that such additional accumulator circuits maymake additional output registers, and/or gain registers, and/or offsetregisters desirable.

Returning to the Computational Sensing Array 300 in FIG. 3, theembodiment shown includes column accumulators 308, magnitude comparators310 and priority encoder 312. The magnitude comparators 310 are coupledto the output registers 366, 368 of the sensing elements 316, 350. Insome embodiments, the magnitude comparators 310 detect if the sensingelement or pixel output is within an acceptance interval. Furtherembodiments detect if the variance of the sensing element or pixeloutput is within an acceptance interval. Further embodiments detect ifthe difference between the two output registers of the sensing elementor pixel of the currently selected row are within an acceptanceinterval. In this manner, changes in sensing element or pixel values canbe detected. High and low thresholds of the acceptance interval areprogrammable on a column by column basis, and/or row by row, and/orsensing element by sensing element or pixel by pixel, in someembodiments. In some embodiments, the high and low thresholds areadjusted automatically based on the sensing element or pixel output or alocal and/or global measure.

Further embodiments contain a time counter in each sensing element orpixel which can be reset when the sensing element or pixel is read, andwhose value can be used to further discriminate which sensing element orpixels are read out. In these embodiments, additional comparators detectif the time counter value is within an acceptance interval. Readout mayrequire both magnitude and time be within acceptable intervals.

The priority encoder 312 sorts the acceptable sensing elements orpixels, e.g., from left to right, and can output column addresses ofacceptable sensing elements or pixels sequentially in one embodiment.The magnitude comparators 310 and the priority encoder 312 thus form anautonomous mechanism, which can alert a CPU (central processing unit) orexternal controllers or devices to readout only sensing elements orpixels of interest, which may represent a small fraction of the totalarray and reduce processing overhead. The column accumulators 308include a column accumulator for each column of the sensing elementarray 306, which sums the intensity of all of the sensing elements orpixels in the column. To prevent overflow, the column accumulators havemore bits than the sensing element or pixel accumulators, in someembodiments. The column accumulators 308 provide information useful foralgorithms requiring average sensing element or pixel intensity whichincludes sensing elements or pixels that are not read out.

Further embodiments sort the sensing elements or pixels according torank rather than in a fixed positional order. In these embodiments, therank is the amount by which the sensing element or pixel output exceededthe threshold. For example, in the case of change detection, the sensingelements or pixels with the greatest amounts of change may have a higheror lower priority than the sensing element or pixel with more modestamounts of change.

Further embodiments of the sensing element or pixel contain circuitry toautomatically remove the sensing element or pixel from consideration bythe autonomous mechanism. For example, a sensing element or pixel maycompare its variance to a threshold and signal the autonomous mechanism(e.g., relevance indexing) to skip it when the threshold is exceeded.

Various combinations of components of the sensing element 350 of FIG. 3can be used in embodiments of the Computational Sensing Array 100 ofFIG. 1A. For example, one embodiment of the conditioning unit 106includes the nonuniformity correction applying the gain register 352 andthe offset register 356 of FIG. 3. The accumulator 362 and associatedlogic may be located in one or both of the collection unit 104 and theconditioning unit 106. Another embodiment of the preprocessing unit 108includes the output registers 366, 368 of FIG. 3. Other embodiments ofthe preprocessing unit 108 include the demultiplexer or selector 364 ofFIG. 3. One embodiment of the readout unit 110 includes the output ports380, 382 of FIG. 3. Further embodiments of the readout unit 110 includebus drivers and other logic associated with reading data.

Various combinations of components of the Computational Sensing Array300 of FIG. 3 can be used in embodiments of the Computational SensingArray 100 of FIG. 1A. For example, the column accumulators 308, themagnitude comparators 310, and the priority encoder 312 of FIG. 3 arelocated above the columns in one embodiment, or have portions thereofintegrated into the preprocessing unit 108 in further embodiments.

In the sensing element 400 shown in FIG. 4, the sensor or detector 402is a photodiode 430, which produces a current 434 in proportion to thearriving photons 432. One embodiment employs a single photon avalanchedetector (SPAD) made from an avalanche diode operating in Geiger mode. Aphotodiode operating in post-avalanche mode may also be used. Othertypes of sensors or detectors can be used in further embodiments ofpixels or sensing elements. For example, further embodiments can usesensors that detect various types of environmental conditions such aselectromagnetic radiation in various ranges across the spectrum, sound,acceleration, ionic concentration, force, pressure, and so on. In theembodiment in FIG. 4, the detector 402 is coupled to a sigma deltamodulator 404 which produces a train of pulses on a sigma delta output426 related to the current 434 through the detector 402. Other types ofsigma delta modulators or other types of analog-to-digital convertersare used in further embodiments.

An embodiment with avalanche photodiodes can have multiple modes ofoperation. In linear mode, below breakdown voltage, photocurrent isintegrated and sensed with the sigma delta modulator. In abovebreakdown, Geiger-mode, a counter is used to count pulses induced bysingle-photon events. A third mode, time-of-flight, can be operated withthe avalanche photodiodes biased in Geiger-mode and an external lightsource. The external light source illuminates an object to be imaged,and the flight time of the photons from light source to reflecting offthe object and being detected by the photodiodes is measured. Themeasurement can be performed with a time-to-digital converter ortime-to-analog converter.

The sigma delta modulator 404 includes an integrator 408 made up of acapacitor and an amplifier 412. The integrator 408 produces a downwardsloping or upward sloping voltage, and the sloping voltage is bufferedthrough the follower 414 and used as an input to the comparator 416. Thecomparator 416 produces a pulse on the comparator output 426 wheneverthe buffered voltage crosses the threshold voltage, thereby producingthe pulse train characteristic of sigma delta converters. The comparatoroutput 426 is fed back through the NAND gate 418 to the one bit digitalto analog converter 410, closing the loop in the sigma delta modulator404. The comparator output 426 is fed forward to the offset/gainregisters 420 and decimator 422, which produce a pixel output 424. Inone embodiment, the offset gain registers 420 and decimator 422 includethe accumulator 362, AND gate 358 and multiplexor 360 of the sensingelement 350 shown in FIG. 3. In further embodiments, other circuitsperforming related functions are included. In the embodiment shown inFIG. 4, the offset/gain registers 420 and the decimator 422 are includedin the nonuniformity correction unit 406 which is coupled to the sigmadelta modulator 404.

In one embodiment, the offset/gain registers 420 of FIG. 4 includes theoffset register 356 and the gain registers 352 from FIG. 3. In anotherembodiment, the decimator 422 of FIG. 4 includes the accumulator 362 ofFIG. 3. In signal processing, it should be appreciated that decimationis the act of dropping samples. As the output of the accumulator 362 issampled once per frame, the accumulator 362 accomplishes the action ofdecimation.

Various components of the sensing element 400 of FIG. 4 can be used inembodiments of the Computational Sensing Array 100 of FIG. 1A. Forexample, one embodiment of the detection unit 102 includes the detector402, more specifically the photodiode 430, of FIG. 4. Another embodimentof the collection unit 104 includes the sigma delta modulator 404 ofFIG. 4. One embodiment of the conditioning unit 106 includes thenonuniformity correction unit 406 of FIG. 4.

FIG. 5 shows circuits 502, 504, 506, 508 for biasing detectors andperforming the one bit digital to analog conversion as shown in FIG. 4.The bucket brigade circuit 502, the switched capacitor circuit 504, thecharge pump circuit 506 and the current feedback circuit 508 areavailable for use with appropriate detectors in embodiments of thesensing element 400. The current feedback circuit 508 switches on a biascurrent for a portion of a clock period to charge the integrationcapacitor, when the comparator output is high. The charge pump circuit506 transfers a charge to the integration capacitor if the comparatoroutput is high, and shunts to ground if the comparator output is low.The switched capacitor circuit 504 alternately switches a capacitorbetween two voltages to emulate the behavior of a resistor, and acts asa charge metering circuit. A second switch is activated if thecomparator output is high. The bucket brigade circuit 502 acts similarlyto the switched capacitor circuit, but couples a capacitor to a controlsignal rather than to ground. Further biasing and digital to analogcircuits and matchings with detectors are readily devised by a personskilled in the art.

FIG. 6A shows a biasing and integrating circuit 600 for use in oneembodiment of the sensing element 400. The circuit operates byautozeroing the offset introduced from the random shift in the voltagethreshold. The circuit reduces the random telegraph signal (RTS) noiseeffect. In this frontend, the photocurrent is integrated on C2 throughamplifier U1 with an open-loop DC gain of A. U1 clamps the voltage at VIto VC so that the diode maintains a constant reverse bias as well asfixing the amount of feedback charge added through capacitor C1 andswitches S1 and S2. Comparator U2 senses when the integratedphotocurrent VT surpasses a threshold set by VR. This will trigger apositive digital pulse, VO. This signal can be used to control switchesS1 and S2 to add a feedback charge onto VI. The amount of feedbackcharge added is Q=C1*(VH−VC). In the sigma-delta architecture, thenumber of times the feedback charge is added can give an estimate to thetotal amount of charge integrated within a given time period. Theaccuracy of this estimate depends on the accuracy of the feedbackcharge, and the accuracy degrades due to random telegraph signal (RTS)noise which is prevalent in CMOS technologies with very small featuresize. The RTS noise is thought to arise from charges randomly trappedand released in the conduction channel of the MOSFET. This leads torandom fluctuations in the threshold voltage, which can be modeled by arandom offset voltage to one of the inputs to the amplifier (VN onpositive input of U1). With this random offset voltage, the feedbackcharge becomes Q=C1*(VH−VC+VN), where VN is a small random voltagefluctuation.

Having discussed the simplified basic architecture for RTS in FIG. 6A,we now describe the operation of the ping-pong frontend. FIG. 6B shows adiagram of the ping-pong frontend designed to mitigate RTS noise.Switches S1-S10 are connected to a two-phase clock where odd and evennumbered switches are controlled by opposite phased clocks, e.g. S1 isclosed (open) and S2 is open (closed) when CLK1 is high (low) and CLK2low (high). Additional switches are added so that U1 can undergo anautozeroing phase; during this phase, the amplifier is disconnected fromC2 and connected in a unity-gain configuration (S6 and S8 open, S7closed). Assuming that the open-loop DC gain A of the amplifier is muchgreater than one, the voltage across C1 is approximately equal to theoffset voltage VN, (A/(1+A))*VN≈VN. After the autozeroing phase, theamplifier is reconnected to C2. The residual offset is equal toVN/A+Qinj/C, where Qinj is charge injection from the switches. Theoffset introduced by the RTS noise is thus proportionally reduced by theamplifier's DC gain. In order to practically obtain a high DC gainwithout utilizing too much area or power in a sensor array, theamplifiers are implemented as regulated cascade inverting amplifiers.

An additional amplifier, U3, is added to integrate the charge on C2while U1 is autozeroing. When U1 is integrating the charge, U3 autozerosand stores its offset on C3. During U1's integration phase, if VT ishigher than the comparator threshold, VR, VO will go high. This signalcan be used to control switches SC1 and SC2. If VO is high, than SC1 isclosed and a feedback charge of Q=C1*(VR−VH) will be added to C2;otherwise, SC2 is closed and no feedback charge is added. In order thatthe feedback be added during the correct phase, U2 is implemented as aclocked comparator that samples VT during U1's autozero phase.

In another embodiment the CSA array may involve multiple transducers perarray unit. FIG. 6C shows the dynamic latched comparator used in thefront-end when two-color integration is desired. For single-colorintegration, the integrate and fire comparator suffices, however itcannot be used for two-color integration where the signal can integrateup or down due to the direction of the positive feedback. Besides beingable to handle two-colors, the dynamic latched comparator is attractivein that it uses minimal power as power is consumed only when thecomparator is making a decision. As the decision process usually occurswithin a couple of nanoseconds, the dynamic power draw is kept low. U1and U2 perform the comparison, U3 and U4 buffer the output of U1 and U2,and U5 and U6 latch the output. Transistors M1-M6 are added to minimizekickback noise on the integration node VT. Instead of sampling VT and VRwith switches, their value is converted to a current that draws chargeoff the output of U1 and U2. When CLK is low, the output of U1 and U2 isreset high, U3 and U4 output low, and M1 and M2 are switched on. Whenclock pulses high, M3 and M4 are switched on, thus M5 and M6 can draincharge off the output of U1 and U2. If VT is greater than the comparatorthreshold, VR, it will hasten the feedback to switch the output of U1 tolow and U2 to high. At this point the comparator has made a decision,and M1 and M2 are shut off. M1 is shut off because the output of U3 goeshigh, and M2 is shut off because its source goes to zero.

Various embodiments of the Computational Sensing Array 100 can be usedto practice a method for sensing. FIG. 7 shows a flow diagram of oneembodiment of a method 700 for sensing in a Computational Sensing Array.In some embodiments of a Computational Sensing Array, each sensingelement or pixel unit of a plurality of sensing elements or pixel unitsin the Computational Sensing Array operates in accordance with themethod 700.

In a block 702, a first analog signal is generated from a detector. Forexample, the detector may be a photodiode, a phototransistor or anothertype of sensor or transducer, and the first analog signal may include avoltage or a current. In a block 704, a frequency modulated signalrepresenting the value of the first analog signal is generated. In oneembodiment, the frequency modulated signal includes a series of pulses,and the pulse rate or frequency of the pulses is related to the amountof light impinging on the detector. In another embodiment, the frequencymodulated signal is produced by a sigma delta modulator coupled to thedetector.

In a block 706, an offset or a gain is applied to the value of thefrequency modulated signal. In one embodiment, the value of thefrequency modulated signal is determined by counting the pulses over aspecified period of time, and the offset or the gain is applied to thecount of the pulses. In another embodiment, the offset is applied byinitializing an accumulator with a fixed point number. The gain may beapplied by adding a fixed point number to the accumulated value in theaccumulator in response to each pulse of the frequency modulated signal.In one embodiment, both offset and gain are applied.

In a block 708, a multibit word representing the value of the frequencymodulated signal with the offset or the gain is generated. In oneembodiment, the multibit word is generated by outputting the accumulatedvalue from the accumulator. In another embodiment, the generation of themultibit word includes incrementing the accumulator with each pulse ofthe frequency modulated signal over the specified period of time. In ablock 710, the current value of the multibit word is stored in a firstregister. The current value of the multibit word is stored while aprevious value of the multibit word is retained in a second register. Inone embodiment, the first and second registers are local to the sensingelement or pixel unit.

Further embodiment of the implementation of the various functionalblocks in state of the art CMOS technology using Near Threshold Voltage(NTV) digital circuits design methodology optimized for minimum energyoperation.

Further embodiments of the implementation of each individualcomputational/communication block in the Computational Sensor Arrayallow for operation at different power supplies ranging from 100 mV tothe maximum voltage allowed by the implementation technology. The powersupply of each block or combination of functional blocks is optimizedfor minimum energy or power-delay product operation. The overallmonitoring of the functionality and optimization of power dissipation inthe array is monitored by the Homeostasis Processor Unit (see FIG. 8).

Further embodiments of the implementation of the CSA involve specialcomputational structures to compensate for component variability that istime-varying. This non-ergodic variability is always present indeep-submicron CMOS technologies. Special local and global computationalstructures may be incorporated to compensate for the variability whichwill manifest itself as time-varying spatial and time-varying temporalnoise, such as random telegraph noise. The homeostasis processor unit isthe supervisor for monitoring and scheduling the compensation oftime-varying variability.

Further embodiments of the implementation of the CSA use CMOS threedimensional technologies (3D CMOS) where each tier of the 3D stack (seeFIG. 1B) is fabricated in a different technology node that is optimumfor the circuits implemented in that particular tier. For example onetier can be in 65 nm CMOS technology and optimized fordiscrete-time/discrete-value and digital circuits while another tierwould be in the 130 nm CMOS node optimized forcontinuous-time/continuous-value and analog circuits.

One embodiment of a time varying compensation is to perform sub-cyclesampling of the signal to estimate a signal flux rate for that cycle andvariance for the sub-cycle samples, store these estimates and compare toconsecutive sub-cycle samples. The deviant sub-cycle samples are set tothe estimated signal flux for that cycle and the relevance ranking isset accordingly. If a number of sub-cycle samples are determined to bedeviant (i.e. exceeding low or high thresholds), the output register(i.e. counter) for the sensing element is set to a preset value and itsrelevance ranking set accordingly. The number of sub-cycle samples usedin estimating the flux and variance, associated thresholds, and thenumber of allowable deviant sub-cycle samples, preset output, andranking are programmable or automatically set values. This compensationcan be employed alone or with other functions stated here.

Another embodiment of a time varying compensation is to replace bad ordefective sensing elements (as determined by the relevance indexing)output or intermediate value with the mean or median of correspondingvalues of neighboring sensing elements. It should be noted andemphasized that this architecture can support frame (or image) basedsensing (i.e. array sensing) or independent single element sensing thatare grouped in an array. The term locally in this document refers to agroup (sub-set) of neighborly (spatially and/or temporally) sensingelements; globally refers to all the operable (as determined by theirrelevance ranking) sensing elements

With the above embodiments in mind, it should be understood that theembodiments might employ various computer-implemented operationsinvolving data stored in computer systems. These operations are thoserequiring physical manipulation of physical quantities. Usually, thoughnot necessarily, these quantities take the form of electrical ormagnetic signals capable of being stored, transferred, combined,compared, and otherwise manipulated. Further, the manipulationsperformed are often referred to in terms, such as producing,identifying, determining, or comparing. Any of the operations describedherein that form part of the embodiments are useful machine operations.The embodiments also relate to a device or an apparatus for performingthese operations. The apparatus can be specially constructed for therequired purpose, or the apparatus can be a general-purpose computerselectively activated or configured by a computer program stored in thecomputer. In particular, various general-purpose machines can be usedwith computer programs written in accordance with the teachings herein,or it may be more convenient to construct a more specialized apparatusto perform the required operations.

The embodiments can also be embodied as computer readable code on acomputer readable medium. The computer readable medium is any datastorage device that can store data, which can be thereafter read by acomputer system. Examples of the computer readable medium include harddrives, network attached storage (NAS), read-only memory, random-accessmemory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes, and other optical andnon-optical data storage devices. The computer readable medium can alsobe distributed over a network coupled computer system so that thecomputer readable code is stored and executed in a distributed fashion.Embodiments described herein may be practiced with various computersystem configurations including hand-held devices, tablets,microprocessor systems, microprocessor-based or programmable consumerelectronics, minicomputers, mainframe computers and the like. Theembodiments can also be practiced in distributed computing environmentswhere tasks are performed by remote processing devices that are linkedthrough a wire-based or wireless network.

Although the method operations were described in a specific order, itshould be understood that other operations may be performed in betweendescribed operations, described operations may be adjusted so that theyoccur at slightly different times or the described operations may bedistributed in a system which allows the occurrence of the processingoperations at various intervals associated with the processing.

The foregoing description, for the purpose of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the embodiments and its practical applications, to therebyenable others skilled in the art to best utilize the embodiments andvarious modifications as may be suited to the particular usecontemplated. Accordingly, the present embodiments are to be consideredas illustrative and not restrictive, and the invention is not to belimited to the details given herein, but may be modified within thescope and equivalents of the appended claims.

What is claimed is:
 1. A method for transducing in a computationalsensing array, comprising: in each sensing element of a plurality ofsensing elements in the computational sensing array: generating a firstcontinuous time, continuous value signal from a transducer; generating amodulated signal representing a value of the first continuous time,continuous value signal in response to receiving the first continuoustime, continuous value signal; applying an offset or a gain to a valueof the modulated signal; generating a vector representing the value ofthe modulated signal with the applied offset or gain; and storing acurrent value of the vector in a first state holding element local tothe sensing element while retaining a previous value of the vector in astate holding element local to the sensing element.
 2. The method ofclaim 1 wherein applying an offset or a gain to a value of the modulatedsignal includes placing the offset into an integrating state holdingelement that increments an accumulated value with each of a plurality ofpulses of the modulated signal.
 3. The method of claim 1 whereingenerating the vector includes counting in response to pulses of themodulated signal with the applied offset or gain, over a specified timespan.
 4. The method of claim 1 wherein applying an offset or a gain to avalue of the modulated signal includes adding a gain value to anaccumulated value with each of a plurality of pulses of the modulatedsignal.